Clarify hosting of SS_ANOVA toolbox and document it as a requirement for GSA

gsa.run for morris==1 requires the SS_ANOVA toolbox. There are two issues:

• The link provided (https://ec.europa.eu/jrc/en/macro-econometric-statistical-software/ss-anova-r-downloads) has been broken for a long time (and has changed in the past). For now, I posted a version I found on my disk at https://forum.dynare.org/t/issue-with-ss-anova-r-link-for-sensitivity-analysis/26248/3 . But this is not a satisfying hosting situation. The package's README states Copyright Marco Ratto, 2008, Joint Research Centre (JRC), which would suggest @rattoma could allow shipping the package directly with Dynare. We need to clarify this.
• The dependency is currently not documented in the manual. If we keep it, we should document it and provide a link to the package.